The isolator would have to be rated pretty high tho I would think. Am I wrong?

Not entirely. You can get high load isolators that can run in a linked mode whereby it's carrying the amperage load on the main battery. Normally though a secondary battery isolator isn't designed for continuous linked operation, rather serving as the charging source for the 2nd battery (which is a low amp load). The secondary battery should be large enough to cater for the anticipated amperage draw (e.g. the compressor) since that is what should take the load.

There seems to be enough resistance even in solid copper, fine strand, power cables to slightly isolate other batteries. We have quite a few customers with dual and/or auxilliary batteries, all in parallel, and the one that's connected to our compressor gets deep cycled and replaced more often than the others.